Many document generating systems convert document data into control signals that operate an ink ejecting print head in a printer, for example, to produce an image of a document with ink drops emitted from the print head. In some of these systems, an electronic version of a document from a personal computer (PC) or other type of computing system is used to produce the document on media, such as paper or film. In other systems, an electronic document is generated by scanning an original hard copy document with a light source to generate reflected light representative of the document. The light signals are converted into electrical signals that may be stored in an electronic memory. The document generating system typically includes an image processor that manipulates the electronic data representing a document to a processed form of the document that is used to produce the hard copy version of the document.
A print engine may be used to manage the subsystems that cooperate to generate a document on media. These subsystems include the image processor and the components that apply or transfer marking material, such as ink, to media to form a document. For example, a direct marking system may include a marking material source, a print head, an image substrate, and a fuser. The marking material source may be an ink cartridge or a solid ink subsystem. Solid ink subsystems have a loader in which sticks of solid ink are loaded and transported to an ink melter that heats the ink sticks to a melting point to generate liquid ink. The liquid ink is collected in a reservoir to supply the print head.
The print head in a document generating system is typically comprised of a plurality of ink jet nozzles arranged in a matrix. The ink jet nozzles are coupled by capillaries to the ink supply. They also include piezoelectric elements that are selectively excited by electrical signals from the print engine to eject ink from the capillaries onto an image substrate. In some systems, the print head may be a single print head supported on a carriage so the print head traverses back and forth in a horizontal path across the face of the image substrate. In other systems, multiple print heads that remain stationary and cover a portion of the image substrate may be used. For example, four print heads, each one covering one quarter of the width of the image substrate, may be mounted on two carriages with each carriage having two print heads. The four print heads are arranged in a staggered two by two matrix opposite the image substrate. Some systems may have one or more print heads that cover the entire width of the image substrate. The carriages are typically movable so the print heads may be moved from a parked or non-imaging position to a print position. In the parked position, the print heads and the imaging member have the greatest separation between them to provide access to the marking unit components. Moving the carriage to the print position brings the print heads proximate the imaging member surface so the heads and the member are separated by a short gap.
Referring to FIG. 1, a side view is shown of a prior art ink printer 100 that corresponds to the description of a printer provided above. As shown in FIG. 1, the ink printer 100 may include an ink loader 96, an electronics module 98, a paper/media tray 92, a print head 50, an intermediate imaging member 52, a drum maintenance subsystem 54, a transfix subsystem 58, a wiper subassembly 60, a paper/media preheater 64, a duplex print path 68, and an ink waste tray 70. In brief, solid ink sticks are loaded into ink loader 96 through which they travel to a melt plate (not shown). At the melt plate, the ink stick is melted and the liquid ink is diverted to a reservoir in the print head 50. The print head 50 includes one or more heaters to help keep the melted ink in a liquid state. The melted ink is ejected by piezoelectric elements to form an image on the intermediate imaging member 52 as the member rotates. Member 52 is called an intermediate imaging member because an ink image is formed on the member and then transferred to media in the transfix subsystem. As shown in FIG. 1, the member 52 is a rotating cylindrical drum. The circumferential surface of the drum is typically manufactured with anodized aluminum.
An intermediate imaging member heater is controlled by a controller to maintain the imaging member within an optimal temperature range for generating an ink image and transferring it to a sheet of recording media. A sheet of recording media is removed from the paper/media tray 92 and directed into the paper pre-heater 64 so the sheet of recording media is heated to a more optimal temperature for receiving the ink image. A synchronizer delivers the sheet of the recording media so its movement between the transfix roller in the transfer subsystem 58 and the intermediate image member 52 is coordinated for the transfer of the image from the imaging member to the sheet of recording media. Sometimes the components that eject ink onto the imaging member, the imaging member, and the components that transfer the image from the imaging member to a media sheet are collectively denoted as a marking unit for a printer.
During the printer manufacturing process, the print heads are among the last components to be installed in the marking unit of the printer to avoid or reduce accidental damage to a print head or drum. After the print heads are installed, the gap between the imaging member and the print head is measured to help ensure the components are within tolerance for the distance that enables accurate placement of ink onto the imaging member. Measurement of this gap and the alignment of the print head with the imaging member is performed with mechanical shim tools or electrical tools, such as a capacitance probe or eddy-current probe. For example, capacitance probes may be mounted to a mask that is attached to the print head. Monitoring equipment provides an excitation voltage to measure capacitances between the probes in the mask on the print head and the imaging member. The measurements obtained from the mask are used to calculate the distance between the print heads and the imaging member. The mask has a limited life arising from the attachment process and the accuracy of the measurement process is subject to the dielectric constant of the air gap, which is affected by the humidity of the air. Additionally, this method is not readily accessible to field technicians who install replacement print heads in printers at customer facilities. Another tool that may be used to measure a gap between an imaging member and a print head is an electronic feeler gauge. Like the capacitive probe mask, this tool does not wear well and is generally unavailable for field installations. More robust methods of measuring the imaging member/print head gap are desirable.